The invention relates to circuits for applying an electromagnetic field to a scanning electron beam for focusing the beam to a point on a screen or other display surface.
Electrons emitted from the cathode of a CRT are accelerated toward the screen, but tend to diverge as they travel due to electrostatic repulsion between the electrons. The electrons can be forced to converge into a tight beam at the screen using electrostatic or magnetic fields. High resolution cathode ray tubes (with very fine spots) generally use magnetic focusing units. Such tubes may be used in television receivers and in various other applications, such as copiers, facsimile machines, projection apparatus and the like, having conventional phosphor screens or fiber optic arrangements.
The focusing device forms a magnetic lens having a focal length. The (X, Y) position of the point of convergence of the beam is varied in a raster pattern by main deflection apparatus apart from the focusing device. However, the screen (or other coupling to an optical system or projection lens) is a substantially planar surface normal to the beam axis rather than a spherical surface, and as a result the distance between the focusing device and the point at which the beam is incident on the screen phosphors or spots varies as a function of the vertical and horizontal scanning position of the beam. The distance from the focusing device to the point of incidence on the screen is greater at the edges of the screen and less in the center. In order to accommodate this difference in beam path length (i.e., to converge the beam at a point as the beam scans through areas of varying path length), the focusing device is driven to produce a magnetic field which varies as a parabola at the scanning rate of the deflection circuits.
The focusing device or magnetic lens can be provided by producing a magnetic field coaxial with the electron beam using a winding which encircles the beam axis. There are different ways in which the field can be produced. According to one technique the magnetic field needed to focus the beam is produced by a circular winding enclosed in a frame, and the current in the winding is varied to produce the required focusing field, including the static and dynamic components thereof. According to another technique the magnetic field is produced by superimposing the fields of a permanent magnet and a dynamic focusing coil. The dynamic focusing coil adds to or subtracts from the field produced by the permanent magnet so that the focal length of the focusing device is shortened or lengthened, respectively.
The current needed in a coil to produce the necessary focusing deflection of the beam as a function of X and Y beam position is parabolic, as defined by the equation: EQU I=K(X.sup.2 +Y.sup.2)
where the current I is proportional to field strength; K is a constant; and X and Y are the coordinates of the spot at which the beam is incident on the screen. The beam axis is the Z axis, perpendicular to both X and Y. Whereas the laws of deviation are linear, the focus correction currents will therefore be parabolic at the respective scanning rates of the electron beam, so as to produce a field which is at maximum at the center of the screen and minimum at the edges. The field can be achieved by providing a current that adds to the field produced by a static field generator, or can be achieved by subtracting from the static field.
The most widely used means for controlling a dynamic focusing coil is to use a current amplifier. A current amplifier is shown in FIG. 5, and the voltage U at the output of the amplifier and the current I which results in the dynamic focusing coil are shown in FIGS. 6(a) and 6(b), representing several horizontal scans and retrace intervals. One of the inputs of an operational amplifier U1 is coupled to a voltage parabola signal through a series resistor R2 and the other input is coupled to a reference (ground). The current coupled through the dynamic focus coil L is sensed using a low value resistor RS in series with the focus coil and ground, and this current signal is fed back through feedback resistor R1. The current amplifier produces a current parabola in the focus coil L in response to the voltage parabola at the input to the amplifier.
A main problem encountered with a current amplifier configuration as shown in FIGS. 5, 6(a) and 6(b) is the dissipation of power. Moreover the amplifier must furnish a high current at high frequency. Scanning using this type of focusing device, especially in projection apparatus, operates between 32 and 128 kHz, requiring a high pass band for the amplifier.
Due to the slope of the required current parabola at the end points, the voltage at the terminals of the coil is quite high. As a result it is necessary to provide a high power supply voltage for the amplifier in order that the necessary voltage can be coupled to the coil. On the other hand, to avoid overvoltage conditions on retrace, the parabolic current signal remains constant during retrace. This means that the current in the output transistors of the amplifier stays at the maximum (FIG. 6(b)) while the voltage varies from its positive to negative peaks (FIG. 6(a)), resulting in additional power loss. Current amplifier configurations of this type may dissipate more than 100W.